1. Field of the Invention
The invention is generally related to constant-force springs and their applications. Particularly, the invention is directed to making constant-force pseudoelastic (superelastic) springs with pseudoelastic alloys and their applications in brush holders for electric machines.
2. Description of the Relevant Art
Pseudoelastic alloys, when stressed, undergo relatively large strains (up to 10%) which would be recovered upon removal of stress. The recoverable strain of pseudoelastic alloys (up to 10%) is far greater than the recoverable strain of conventional metals (e.g., about 0.3% for steel). A major fraction of pseudoelastic strain occurs under a relatively constant level of stress. Recovery of pseudoelastic strains during stress removal also largely takes place under a relatively constant level of stress. This is unlike elastic strain where during elastic strain occurrence and recovery stress varies in proportion (linearly) with strain. A schematic presentation of the stress-strain relationships for pseudoelastic alloys and elastic materials during loading and unloading is given in FIG. 1.
Pseudoelasticity is exhibited by shape memory alloys within a particular range of temperature. This temperature range can be adjusted to suit particular service conditions of targeted applications through tailoring the composition and processing of alloys. Examples of shape memory alloys exhibiting pseudoelastic behavior include nickel-titanium alloys and copper-based alloys such as Cuxe2x80x94Znxe2x80x94Al and Cuxe2x80x94Alxe2x80x94Ni.
Shape memory alloys have been used in different spring applications. U.S Pat. No. 4,846,729 to Hikami et al., U.S. Pat. No. 4,952,162 and U.S. Pat. No. 5,059,133 to Hikami et al. disclose electronic connectors with a shape memory spring which transmits a recovery force when the shape memory spring reaches or exceeds its transformation temperature. U.S. Pat. No. 4,848,388 to Waldbusser discloses a self actuating valve system where a shape memory spring applies the actuating force when temperature exceeds its transformation temperature (e.g. due to fire). U.S. Pat. No. 5,014,520 and U.S. Pat. No. 5,083,439 to Omer et al. disclose a control device with a shape memory spring which moves a valve when temperature is raised above its transformation temperature. U.S. Pat. No. 5,217,382 to Sparks discloses en electric receptacle with a shape memory spring which is heated above its transformation temperature in order to cause certain movements caused by its shape recovery. World Intellectual Property Organization No. 9841962A2 to Schleppenbach et al. discloses an apparatus using the actuating effect associated with shape recovery of shape memory springs upon heating above their transformation temperature. Japanese Pat. No. 40766274A2 to Sho et al. discloses a shape memory spring of honeycomb-like geometry which acts as an actuator. Japanese Pat. No. 60070153A2 to Katsuji discloses a shape memory spring of particular geometry which acts as an actuator controlled by temperature-change. Japanese Pat. No. 6109049A2 to Kiyosli discloses a superelastic spring of particular geometry which exhibits shape memory (actuating) effect and excellent durability.
The shape memory springs which are subject of the above inventions are essentially heat-activated actuators. The pseudoelastic spring which is subject of this application is distinguished from the above shape memory springs because it is still a spring (and not a heat-activated actuator) with novel geometry and pseudoelastic pre-straining condition, which exhibits a particular force-deformation (i.e., constant-force) behavior.
Japanese Pat. No. 58217834A2 to Akira et al. disclose a superelastic spring subjected to plastic deformation so that a permanent set of more than 10% remains upon unloading. This process yields a (conventional) linear spring which is relatively stable over a wide temperature range. Japanese Pat. No. 60009864A2 to Kazuo et al. discloses a superelastic spring of conventional (linear) behavior with a relatively high (recoverable) deformation capacity. Japanese Pat. No. 61084361A2 to Kiyoshi et al. discloses the manufacturing process of a pseudoelastic spring of high flow stress near the body temperature. Japanese Pat. No. 7062506A2 to Hiroshi discloses production of a superelastic spring of conventional (linear) behavior with high (recoverable) deformation capacity. Japanese Pat. No. 7062505A2 to Hiroshi discloses a superelastic spring of conventional (linear) behavior with excellent fatigue characteristics.
The superelastic (pseudoelastic) springs discussed above all act as conventional (linear) springs with forces varying proportionally with deformations. The pseudoelastic spring disclosed in this invention is distinguished from the above by its particular spring by its novel geometry and pseudoelastic pre-straining condition which yield a constant-force behavior where the spring force is relatively constant over large deformations; this deviated from the conventional (linear) behavior of the above superelastic springs where force varies proportionally with deformation.
The constant-force pseudoelastic spring which is subject of this invention can be used in constant-force brush holders for electric machines. Examples of constant-force brush holders which utilize conventional materials (and not pseudoelastic alloys) are disclosed in U.S. Pat. No. 4,389,588 to Rankin, U.S. Pat. No. 5,463,264 to Koenitzer, and U.S. Pat. No. 5,907,201 to Peot et al.
It is an object of this invention to provide pseudoelastic springs having a constant force behavior over relatively large deformations.
It is another object of this invention to provide particular geometric configurations in pseudoelastic springs which cause a constant-force behavior.
It is another object of this invention to provide particular pseudoelastic pre-straining conditions in pseudoelastic springs which cause a constant-force behavior.
It is another object of this invention to provide constant-force brush holders which incorporate pseudoelastic springs.
Applicant has developed a novel geometry and pseudoelastic pre-straining condition which undergo relatively large deformations at a relatively constant level of force. Pseudoelastis springs with various versions of such geometric and pre-straining conditions have been manufactured and tested. The results validated the constant-force behavior of such springs.
According to the invention, there is provided springs made of pseudoelastic alloys, with particular geometric and pre-straining conditions which exhibit a constant-force behavior.